Now backup and restore operations free logger resources calling
to the destroy() method. This commit ensures:
1) That gc_collect_cycles() is not used anymore in backup-related tests.
2) That all backup and restore controllers in test do always call
to the detroy() method.
3) Some unset() calls, needed to make gc_collect_cycles() are not used
anymore.
Unit tests were failing on MSSQL. gc_collect_cycles() was
removed from the phpunit utils.php file to save time in running
the tests, but MSSQL doesn't clean up open files as well as
other databases.
This patch includes the garbage collection for the unit tests
that require it.
Implements backup and restore for the new conditional availability
data. This includes:
* Backup and restore of new field.
* Restore updates IDs in conditions, as required.
* Restore converts availability data from legacy (Moodle 2.6) backups.
* Unit tests for all this.
